This invention relates generally to a liquid ebonite coating. More particularly, it relates to a liquid ebonite coating containing two reactive components.
There is a pervasive and continuing need for protecting metals from corrosive chemical action, such as in metal pipes, stacks, chimneys, bridges, chemical plant constructions, ship hulls, and containers for aggressive chemicals, to name just a few applications. In addition to having a high resistance to chemical action, an ideal coating has certain other properties: the raw materials required to produce the coating are commercially available, inexpensive and non-hazardous; the coating has the ability to be easily applied to the metal, e.g. by spraying, spreading, or free casting; the coating has strong adhesion to many different metals; it is strong, hard, abrasion resistant and thermostable; and the hardening process of the coating can be performed in contact with moisture, does not require extreme or long heating, and does not release toxic fumes. An ideal metal coating may have many additional properties, depending on the particular application or purpose of the coating.
The most widespread anticorrosive coatings possessing many of the above properties are polyurethanes and epoxide resins (see for example, Coating Systems: A guidance Manual For Field Surveyors, American Bureau of Shipping and Affiliated Companies, 1995). These coatings have good chemical resistance to many substances, have adhesion to metals that is satisfactory for many purposes, and have good mechanical properties. Neither polyurethanes nor epoxide resins, however., satisfy all the criteria for an ideal coating for metal. In particular, although polyurethanes have outstanding oil-gasoline resistance, a unique combination of favorable physical-mechanical properties, and strong adhesion to some metals, they are not stable under elevated temperature, alkaline hydrolysis, and persistent tension. Epoxide resins, although they have outstanding adhesion to some metals, do not have a satisfactory resistance to acids, certain solvents, temperature changes, and vibration. One of the most significant problems associated with both epoxide resins and polyurethanes is their susceptibility to underfilm corrosion associated with defects in the coating surface. Because these coatings are bonded to the metal only by adhesive bonding, these bonds can be broken by the introduction of moisture, solvents or other substances.
As is known from rubber chemistry (Encyclopedia of Polymer Science and Technology, John Wiley and Sons, N.Y., vol 12, p.161, 1970), solid ebonite, commonly known as hard rubber, is a polymer material with sulfur content used for vulcanization. Ebonite, like elastomeric or flexible rubber, is made from a combination of sulfur with polydienes (unsaturated rubbers containing double bonds). The sulfur and polydienes are combined with some auxiliary additives and heated to produce vulcanization. Typical mass ratios of sulfur to rubber are 2:100 for elastomeric rubber and 40:100 for hard rubber. Due to the large degree of sulfide cross-linking formed in the vulcanization process, solid ebonite is a hard, non-flexible, plastic-like material possessed of unique chemical resistance to aggressive substances such as acids, alkalis, salt solutions, oil, and gasoline. In addition, solid ebonite has good mechanical properties. Consequently, these conventional rubbers are commonly used as materials for fuel tanks, containers for aggressive substances, and other applications. In spite of these advantages, however, solid rubbers can not be easily applied to metal surfaces, they release toxic fumes during vulcanization, and they require a long time to harden.
More than 30 years ago liquid rubbers were synthesized. (See Alan R. Luxton, xe2x80x9cThe Preparation, modification and application of non-functional liquid polybutadienesxe2x80x9d, Rubber Chemistry and Technology, 54 (1981) 3, 596-626.) Like earlier rubbers, liquid rubbers are formed from compounds such as polybutadiene, polyisoprene, butadiene-styrene, and butadiene-nitrile. In contrast to the hard rubbers, which are made from such compounds having molecular weights on the order of 100,000 to 500,000, the liquid rubbers are made from such compounds having molecular weights of only 2,000 to 4,000. Consequently, the low molecular rubbers permit castable processing by pouring, spreading, spraying, or rolling, while providing similar properties as the hard rubbers after curing. Liquid rubber, therefore, may be used to more easily coat metal surfaces.
However, all the prior art liquid ebonite coatings suffer from two major disadvantages. First, during the heating, especially on a vertical surface, the coating will have problems of sagging, flowing or dripping since the viscosity of the coating decreases as the temperature increases. Therefore, their viscosity must be increased to prevent sagging. The high viscosity makes spraying of the liquid ebonite mixture very difficult, and even impossible in some cases. Second, the coatings are gooey, which makes the handling or inspection of the coating before vulcanization impractical. The coating of large equipment, such as a precipitator, requires a tack free surface so that coated parts can be handled and assembled before the whole equipment is heated and vulcanized. Also, critical coatings such as tank linings must be inspected to ensure even coating thickness and holiday free coating. Inspection requires tack free surface so that an inspector can touch or walk (for a large structure) on the coated surface.
Liquid ebonite mixture (LEM) compositions are disclosed by Figovsky in WO 0,006,639 issued Feb. 10, 2000, which contains 10% of a high molecular weight rubber for increasing the viscosity of the liquid ebonite mixture for preventing the problems of sagging. Unfortunately, the high viscosity LEM of Figovsky is unsprayable. Furthermore, LEM of Figovsky is gooey, which makes the handling and inspection of the coating before vulcanization impractical.
A liquid rubber based ebonite coating has been disclosed by Rappoport in U.S. Pat. No. 5,766,687 issued Jun. 16, 1998 and U.S. Pat. No. 5,997,953 issued Dec. 7, 1999. In these prior art patents, to prevent sagging, the viscosity of the liquid ebonite mixture is increased by adding thixotropic fillers, such as bentonites and fume silica, or high structure carbon black. The ebonite coating in Rappoport""s inventions includes a single component with the compositions shown in Table 1.
Unfortunately, the single component ebonite coating of Rappoport sags and is unsprayable.
There is a need, therefore, for a liquid ebonite mixture for coating, which will overcome the disadvantages of the prior art, but still maintain excellent properties, such as chemical resistance and tenacious bonding to metal.
These objects and advantages are attained by a two-component reactive liquid ebonite mixture.
According to an exemplary embodiment of the present invention, a liquid ebonite mixture for coating contains first and second components. The first component contains first unsaturated polymers, which include first functional groups that are capable of reaction at ambient temperature either with or without a catalyst, a vulcanization activator, and a vulcanization agent. The second component includes second unsaturated polymers, which contain second functional groups that will react with the first functional groups of the first unsaturated polymers at ambient temperature. The first and second unsaturated polymers must contain sufficient unsaturation in the backbones for forming linkages with the vulcanization agent. It is preferable that the polymer backbone be polybutadiene. However, polyisoprene, poly(butadiene-co-acrylonitrile), poly(isobutyl-co-isoprene) or poly(ethylene-co-propylene-co-diene) that contain at least 10 mole % of diene unsaturation can also be used. The mass parts of the first and second unsaturated polymers in the mixture are 50.
The preferable functional groups for the first unsaturated polymers are hydroxyl, epoxy, acrylic, and their combinations. In addition, the first unsaturated polymers can also be functional unsaturated polymers that are partially epoxidized.
The preferable functional groups for the second unsaturated polymers are isocyanate and maleic anhydride. Alternatively, the second unsaturated polymers can be a toluene diisocyanate terminated unsaturated prepolymer or 4,4xe2x80x2-methylene diphenyldiisocyanate terminated unsaturated prepolymer. The second unsaturated polymers must be thermodynamically compatible with the first unsaturated polymers so that macroscopic separation will not occur, and the sulfur vulcanization can happen homogeneously through out the coating.
Sulfur is the main vulcanization agent, and its mass parts is approximately 15-50, preferably 30-50. It is preferable that it has fine particle size so that the dispersion will be easier. The vulcanization activator is zinc oxide, and its mass parts is approximately 5-35. However, zinc stearate can also be used as the vulcanization activator.
The first and second components further include first and second viscosity reducers for adjusting the viscosity of the liquid ebonite mixture. The first and second viscosity reducers typically contains unsaturated polymers, which are the same or similar with the first and second unsaturated polymers, and which must have viscosity lower than the viscosity of the first and second unsaturated polymers respectively. The first and second viscosity reducers may contain no functional groups, or they may contain functional groups that are the same or similar with the first and second functional groups respectively so that each component remains non-reactive at ambient condition until two components are mixed together. The mass parts of the first and second viscosity reducers are approximately 0-30.
Furthermore, fume silica or bentonite is added into the first component as a thixotropic agent, and the amount should be kept as low as possible so that the mixture can be sprayed easily. The mass parts of the thixotropic agent is approximately 0-10. In addition, the first component can contain carbon black, which is used as a reinforcing agent, colorant and UV stabilizer. The amount of the carbon black also should be minimized so that it does not increase the viscosity of the mixture drastically. Typically, the mass parts of the reinforcing agent is approximately 0-10.
The first component, preferably but not absolute necessarily, further contains a sulfur solubilizer. The sulfur solubilizer contains polyamine or polyamide amine with mass parts approximately 1.5-6. In addition as serving as a solubilizer, polyamine is also reactive to the isocyanate groups in the second unsaturated polymers to form polyurea linkages, which accelerates the gelation process. The polyamine in the first component can be substituted by butadiol, a chain extender. Furthermore, a urethane catalyst, such as dibutyl tin dilaurate (DBTDL), is also added into the first component with mass parts approximately 0-3. Alternatively, other urethane catalysts, such as K-KAT 348, a Bismuth carboxylate based compound, can also be used.
A vulcanization accelerator with mass parts approximately 1-7 is additionally added into either first or second components. Tetramethylthiuram disulfide, such as Methyl Tuads, is a preferable vulcanization accelerator because it causes the mixture to gel and become tack free quickly, and the mixture is also can be vulcanized at lower temperature or in shorter time. However, Methyl Tuads is only added into the second component since it works synergistically with polyamine, which is contained in the first component, to accelerate the sulfur vulcanization that makes the sulfur curing feasible at 80xc2x0 C. Therefore, at ambient temperature, if Methyl Tuads is contained in the first component with polyamine, a gelation process will occur. Thus, to maintain sufficient shelf life, Methyl Tuads must be separated from polyamine and is mixed into the second component. Other similar thiuram, such as tetrabutylthiuram disulfide, tetraisobutylthiuram disulfide, tetrabenzylthiuram disulfide, 2-mercaptobenzothiazole, benzothiazyl disulfide, oxydiethylenebenzothiazole-2-sulfenamide, N-cyclohexyl-benzothiazole-2-sulfenamide, N-tert-butyl-2-bemzothiazolesulfenamide, can also be added in the second component as a vulcanization accelerator. Dyphenylguanidine (DGP) is also used as a vulcanization accelerator. Since DGP can be mixed with polyamine in the first component without causing a shelf life problem, it can be added in the first component. However, using DGP as a vulcanization accelerator the gelation and tack free time are longer.
The first and second components are first formed separately by mixing their compositions. The two components are then simultaneously mixed together with a mass ratio of between about 0.75 and about 2.75 and sprayed onto a metal substrate. Upon mixing and spraying, the first functional group of the first unsaturated component will react with the second functional group of the second unsaturated polymers, optionally with the aid of tin catalyst, at ambient temperature and gel to a non-gooey rubbery state. The gelation prevents coating from flowing, particularly on a vertical surface, to ensure an even coating thickness. The coating is then heated to elevated temperature for sulfur vulcanization. The vulcanization condition can vary from 180xc2x0 C. for 30 minutes to 80xc2x0 C. for three days, depending on the selection of vulcanization accelerator and process temperature. During the heating, especially on a vertical surface, the coating will not drip or sag due to the gelation. After the vulcanization, the coating is further hardened to a tough surface. The coating possesses excellent adhesion to steel and exhibits outstanding chemical resistance.
The above embodiments provide a liquid ebonite coating that is readily sprayable. Such a liquid ebonite coating may be set up quickly without sagging and have a controllable thickness.
Furthermore, the above embodiments provide a liquid ebonite coating that has low tackiness that allows easy handling and inspection before vulcanization.
Although the following detailed description contains many specifics for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the following preferred embodiment of the invention is set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.
In an exemplary embodiment of the present invention, a liquid ebonite mixture for coating surfaces contains a two-component reactive liquid elastomer. The formulation of the ebonite coating is prepared by mixing first and second components. The first component typically includes first unsaturated polymers, a vulcanization activator of zinc oxide, and a vulcanization agent. Sulfur is the main vulcanization agent. It is preferable that sulfur has fine particle size so that the dispersion will be easier. The first component further includes a thixotropic agent of fume silica or bentonite, a first viscosity reducer for adjusting the viscosity of the liquid ebonite mixture, and optionally a solubilizer for the vulcanization agent. The solubilizer preferably contains polyamine or polyamide amine. Alternatively, a chain extender of butandiol can be used to replace polyamine. Carbon black is used as a reinforcing agent, colorant and UV stabilizer. The amount of the reinforcing agent and the thixotropic agent should be minimized so that the viscosity of the ebonite coating does not increase drastically, so the liquid ebonite mixture can be sprayed easily. A urethane catalyst, such as dibutyl tin dilaurate (DBTDL), is also added into the first component. Alternatively, other urethane catalysts, such as K-KAT 348, a Bismuth carboxylate based compound, can also be used.
The selection of the first unsaturated polymers in the first component has to satisfy two requirements. First, the first unsaturated polymers must contain first functional groups that are capable of reaction at ambient temperature, either with or without a catalyst. The preferred functional groups are hydroxyl, epoxy, acrylic, and their combinations. Second, for vulcanization, the first unsaturated polymers must contain sufficient unsaturation in the backbones for forming sulfur linkages. It is preferable that the first unsaturated polymers contain backbones of polybutadiene. However, polyisoprene, poly(butadiene-co-acrylonitrile), poly(isobutyl-co-isoprene) or poly(ethylene-co-propylene-co-diene), which contain at least 10 mole % of dien unsaturation can also be used.
The second component typically includes second unsaturated polymers. The selected second unsaturated polymers must satisfy three requirements. First, the polymers must contain second functional groups, such as isocyanate or maleic anhydride, which will react with the first functional groups of the first unsaturated polymers at ambient condition. In addition, polyamine mixed in the first component is also reactive to the isocyanate groups in the second unsaturated polymers to form polyurea linkages, which accelerates the gelation process. Second, the polymers must contain sufficient unsaturated backbones to allow vulcanization with sulfur. Third, the polymers must be thermodynamically compatible with the first unsaturated polymers of the first component so that macroscopic phase separations will not occur and the sulfur vulcanization can happen homogeneously throughout the coating. It is preferable that the second unsaturated polymers contain backbones of polybutadiene. However, polyisoprene, poly(butadiene-co-acrylonitrile), poly(isobutyl-co-isoprene) or poly(ethylene-co-propylene-co-dien), which contain at least 10 mole % of diene unsaturation can also be used. The second component further contains a second viscosity reducer for adjusting the viscosity of the liquid ebonite mixture.
The first and second viscosity reducers in the first and second components typically contain unsaturated polymers similar to those used for the first and second unsaturated polymers (i.e., polybutadiene, polyisoprene, poly(butadiene-co-acrylonitrile), poly(isobutyl-co-isoprene), or poly(ethylene-co-propylene-co-dien.) The first and second viscosity reducers must have a lower viscosity than the first and the second unsaturated polymers. The first and second viscosity reducers may not contain functional groups. Otherwise, the first and second viscosity reducers may contain the same or similar functional groups as the first and second functional groups respectively so that the polymers and the viscosity reducer in each component remain non-reactive at ambient condition until the first and second components are mixed together. The first and second viscosity reducers also preferably contain sufficient unsaturation for sulfur vulcanization and must be thermodynamically compatible with the first and second unsaturated polymers in the first and second components. For example, the first viscosity reducer can be a lower molecular weight non-functional or hydroxyl functional liquid polybutadiene, polyisoprene, butadiene nitrile rubber. Liquid butyl or EPDM rubber, which contain at least 5 mole % of unsaturated monomer, can also be used.
Furthermore, either first or second component includes a vulcanization accelerator. Tetramethylthiuram disulfide, e.g., Methyl Tuads, is a preferred vulcanization accelerator because it gels and becomes tack free quickly, and can be vulcanized at lower temperature or in shorter time. However, polyamine works synergistically with Methyl Tuads to accelerate the sulfur vulcanization, which makes the sulfur curing feasible at 80xc2x0 C. At ambient temperature, if Methyl Tuads is mixed in the first component with polyamine, gelation will occur in the first component. To maintain sufficient shelf life, Methyl Tuads must be separated from polyamine, and therefore it is mixed in the second component. Alternatively, other thiurams, such as, tetrabutylthiuram disulfide, tetraisobutylthiuram disulfide, tetrabenzylthiuram disulfide, 2-mercaptobenzothiazole, benzothiazyl disulfide, N-oxydiethylenebenzothiazole-2-sulfenamide, N-cyclohexyl-benzothizole-2-sulfenamide, N-tert-butyl-2-benzothiazolesulfenamide, can also be used. In addition, diphenylguanidine can be used as a vulcanization accelerator. It was observed that diphenylguanidine (DPG) can be mixed with polyamine in the first component without causing a shelf life problem. However, with DPG as the vulcanization accelerator, the gelation and tack free time are longer, and it requires at least two hours at 160xc2x0 C. to complete the vulcanization.
The first and second components are formulated separately by mixing their composition. A liquid ebonite formulation for coating metals is prepared by simultaneously mixing and spaying the first and second components onto a metal substrate. The first and the second components are mixed with a ratio of between about 0.75 and about 2.75. Upon the mixing and spraying, the first functional groups in the first unsaturated polymers will react with the second functional groups of the second unsaturated polymers at ambient temperature, optionally with the aid of a catalyst, and the mixture gels to a non-gooey rubbery state. At this stage, an inspector can examine the coating to detect any holidays or imperfections. Any remedial actions such as additional coating can be applied to ensure coating reliability. By properly adjusting the degree of gelation to render the surface tack free, the coated parts can also be handled for assembly without getting hands messy.
After inspection and assembly, the coating is then heated to elevated temperature for sulfur vulcanization. The vulcanization condition can vary from 180xc2x0 C. for 30 minutes to 80xc2x0 C. for three days, depending on the selection of vulcanization accelerator and process temperature. During the heating, especially on a vertical surface, the coating does not sag or drip.
After the sulfur vulcanization, the coating is further hardened to a tough surface, which typically has hardness above 70 Shore D. The coating possesses excellent adhesion to steel and exhibits outstanding chemical resistance, typically of all ebonite compositions.
Tables 2a and 2b below show preferable compositions of first and second components in the liquid ebonite mixture according to an exemplary embodiment of the present invention. The mass parts of a compound is number of parts by mass of the compound in the mixture.
Both first and second components can optionally contain additional fillers, flame retardants, color pigments, age resistors (e.g., antioxidants and antiozonants), processing oils and solvents.
To supplement the foregoing disclosure, the following examples are provided to illustrate specific aspects of the invention and particular techniques useful for making various coatings according to the present invention. However, it is to be understood that the examples are for illustrative purposes only and in no manner is the present invention limited to the specific disclosures therein.